Epicyclic gear systems used in aircraft propulsion comprise a sun gear; a ring gear; and a number of planet gears interposed between the sun gear and the ring gear and fitted to a planet carrier by respective connecting pins and with the interposition of sliding or rolling bearings.
A typical planet carrier configuration is asymmetrical, i.e. comprises two substantially plate portions, which are positioned facing each other on opposite axial sides of the planet gears, are connected integrally to each other by a number of cross members or tenons, and only one of which is connected integrally to a member stationary or rotary, depending on the configuration of the gear system—which reacts to the torque transmitted from the planet gears to the planet carrier.
As stated, each planet gear is connected to the planet carrier by a respective connecting or supporting pin, the opposite ends of which are connected—normally, though not necessary, locked—to the plate portions, and an intermediate portion of which supports a sliding or rolling bearing, e.g. with two sets of cylindrical rollers.
In the application considered, the planet gear bearings—regardless of whether they are sliding or rolling types—fail to allow for misalignment between the planet gear and respective supporting pin.
During operation of the gear system, the forces transmitted from the planet gears to the planet carrier deform the planet carrier and, in particular, result in relative rotation of the two plate portions.
Unless steps are taken to prevent it, this rotation deforms both the tenons and the supporting pins, the axes of which go from a rest condition, in which they are parallel to the axes of the sun gear and ring gear, to a work condition, in which they form, with the sun gear and ring gear axes, an angle of other than zero, and which varies, depending on the intensity of the forces transmitted and therefore on the degree of deformation of the planet carrier.
The deviation of the supporting pin axes, and therefore of the respective planet gear axes with respect to the ring gear and sun gear axes, results in uneven load distribution on the meshing teeth of the planet, sun and ring gears, and on the bearings, thus impairing operation of the gear system and normally significantly reducing the working life of its component parts.
Various solutions have been proposed to eliminate these drawbacks. A first consists in modifying the geometry of the component parts, and in particular of the bearing work seats, to compensate the effects of deformation under load; and a second consists in equally distributing the rigidity of the planet carrier and/or supporting pins to eliminate the effects of relative rotation of the plate portions on the bearing work seats.
Whereas the first only eliminates the effects of deformation at a given transmitted load value, the second effectively compensates the problem regardless of load.
One known solution, in accordance with the second solution and described, for example, in the Applicant's Patent U.S. Pat. No. 6,409,414, is to employ asymmetric supporting pins, i.e. with portions differing in rigidity from one axial end to the other. Though effective in balancing the effects of deformation under load, this solution calls for locally compensating not only the effects of the variation in rigidity along the axes of the supporting pins on load distribution on the bearing, but also asymmetrical centrifugal action on the planet carrier.
Another solution, in accordance with the second solution, is to provide weight-reducing slots around the pin supports on the plate portion connected to the torque reaction member, as described, for example, in Patent WO2009102853A1.
This solution has the advantage of allowing use of symmetrical pins, but involves severe stress on the plate portion, especially in applications involving severe loads and deformation.